Flue gas discharged from coal combustion boilers used as combustion apparatuses in, for example, thermal power plants and other facilities contains highly toxic mercury. Therefore, various systems for reducing the amount of mercury in the flue gas have conventionally been studied.
Generally, a coal combustion boiler includes a wet desulfurization unit for reducing the amount of the sulfur content in flue gas. In an air pollution control facility including a boiler provided with such a desulfurization unit used as an air pollution control apparatus, it is well known that the ratio of water-soluble divalent metallic mercury (Hg) increases as the content of chlorine (Cl) in the flue gas increases. In this case, the mercury is easily collected by the desulfurization unit.
Therefore, in recent years, various proposals have been made on methods and apparatuses for treating metallic mercury using a combination of a denitration catalyst layer for reducing NOx and a wet desulfurization unit that uses an alkali absorbent as a sulfur oxide (SOx) absorbent.
Known examples of the method of treating metallic mercury in flue gas include a method that uses an absorbent such as activated carbon or a selenium filter to reduce the amount of the metallic mercury. However, this method requires special absorption-reduction unit and therefore is not suitable for treatment of a large volume of flue gas such as flue gas treatment in power plants.
In one proposed method of treating metallic mercury in a large volume of flue gas (see, for example, Patent Literatures 1 and 2), a chlorinating agent is gas-atomized into a flue gas duct in a process upstream of a high-temperature denitration catalyst layer to oxidize (chlorinate) the mercury on the denitration catalyst. Then the water-soluble chlorinated mercury formed is absorbed in a downstream wet desulfurization unit. The apparatus and technique for gas-atomization into a flue gas duct have been in practical use, for example, spraying of NH3 onto a denitration catalyst layer or gas-atomization of a chlorinating agent.
FIG. 5 is a schematic diagram of an air pollution control system for a coal combustion boiler. As shown in FIG. 5, the conventional air pollution control system 100 includes: a denitration catalyst layer 13 for reducing the amounts of nitrogen oxides (NOx) in flue gas 12 from a coal combustion boiler 11 to which coal is supplied as fuel F and for oxidizing mercury (Hg) with hydrochloric acid (HCl) sprayed into the flue gas 12; an air preheater 14 for recovering heat of the flue gas 12 in which nitrogen oxides (NOx) have been reduced in amounts; an electric precipitator 15 for reducing the amount of soot particles in the flue gas 12 from which the heat has been recovered; a desulfurization unit 16 for reducing the amounts of sulfur oxides (SOx) and mercury (Hg) in the flue gas 12 in which the soot particles have been reduced in amount; and a stack 18 for discharging the desulfurized flue gas 12 as cleaned-up gas 17.
A flue gas duct 19 on the upstream of the denitration catalyst layer 13 has an injection section for hydrochloric acid (HCl), and hydrochloric acid (liquid) stored in a hydrochloric acid (liquid HCl) supply unit 20 is vaporized in a hydrogen chloride spraying unit 21 and then sprayed into the flue gas 12 as hydrogen chloride through hydrogen chloride (HCl) spray nozzles 21a. 
The flue gas duct 19 on the upstream of the denitration catalyst layer 13 also has an injection section for ammonia (NH3), and ammonia supplied from an NH3 supply unit 29 is sprayed into the flue gas 12 through ammonia spray nozzles 29a to reduce nitrogen oxides (NOx).
In FIG. 5, reference sign 25 represents an oxidation-reduction potential measuring-controlling unit (OPR controller), and 26 represents air.
The flue gas 12 from the coal combustion boiler 11 is supplied to the denitration catalyst layer 13. Then, air 27 is heated in the air preheater 14 by heat exchange, and the resultant flue gas 12 is supplied to the electric precipitator 15 and then to the desulfurization unit 16 and discharged to the air as the cleaned-up gas 17.
In addition, to reduce the influences of, for example, corrosion damage to the apparatus caused by the chlorinating agent to thereby improve reliability, the concentration of mercury in the flue gas after wet desulfurization is measured by a mercury monitor to adjust the supply amount of the chlorinating agent on the basis of the mercury concentration after desulfurization (see, for example, Patent Literature 2).
As described above, in the conventional system, hydrogen chloride and ammonia are supplied to the flue gas 12 to reduce the amounts of NOx (nitrogen oxides) in the flue gas 12 and to oxidize mercury (Hg) in the flue gas 12.
More specifically, NH3 is used for reduction-denitration of NOx. NH3 supplied from the NH3 supply unit 29 is sprayed into the flue gas 12 through the ammonia (NH3) spray nozzles 29a to denitrate the flue gas 12 by converting NOx into nitrogen (N2) in the denitration catalyst layer 13 through the reduction reactions represented by the following formulas:4NO+4NH3+O2->4N2+6H2O; and  (1)NO+NO2+2NH3->2N2+3H2O.  (2)
Hydrogen chloride is used to oxidize mercury. Hydrogen chloride used as the chlorinating agent is supplied from the liquid HCl supply unit 20 to the hydrogen chloride (HCl) spray unit 21, and the hydrochloric acid vaporized therein is sprayed into the flue gas 12 as hydrogen chloride (HCl) through the hydrogen chloride spray nozzles 21a. Low-solubility Hg is oxidized (chlorinated) on the denitration catalyst in the denitration catalyst layer 13 according to the following formula to convert Hg into mercury chloride (HgCl2) having high water solubility. Then Hg contained in the flue gas 12 is reduced in amount in the desulfurization unit 16 disposed downstream of the denitration catalyst layer 13.Hg+2HCl+½O2->HgCl2+H2O  (3)
When coal or heavy oil is used as fuel, the fuel contains Cl, and therefore the combustion gas contains Cl. However, the Cl content varies depending on the type of the fuel, and it is difficult to control the concentration of Cl in the flue gas 12. Therefore, preferably, HCl or the like is added upstream of the denitration catalyst layer 13 in an amount more than necessary to reduce the amount of Hg in a reliable manner.
In the denitration catalyst layer 13 used, the denitration catalyst is supported on a honeycomb-shaped substrate having rectangular passages arranged in a lattice pattern, and the cross-sections of the passages have a polygonal shape such as a triangular or rectangular shape.